G-protein coupled receptors (GPCRs) constitute a major class of proteins responsible for transducing a signal within a cell. Upon binding of a ligand to an extracellular portion of a GPCR, a signal is transduced within the cell that results in a change in a biological or physiological property of the cell. GPCRs, along with G-proteins and effectors (intracellular enzymes and channels modulated by G-proteins), are the components of a modular signaling system that connects the state of intracellular second messengers to extracellular inputs.
GPCR genes and gene-products are potential causative agents of disease. For example, specific defects in the rhodopsin gene and the V2 vasopressin receptor gene have been shown to cause various forms of retinitis pigmentosum, and nephrogenic diabetes insipidus. These receptors are important to both the central nervous system and peripheral physiological processes.